Leading/Retreating Rotor Blades
 

I have been pondering a question for a few weeks now and wonder if any of you very qualified Rotorheads can explain to me the following:- in any helicopter we know that the rotor blades are advancing and at the same time retreating, if a helicopter is flying into on the nose headwind say for example 25knts, if you think of the disc half of it is advancing whilst the other half is retreating, now then add to that the wind velocity of eg 25knts what effect will that have on the retreating blade, will it:-
A. try to push it faster through its cycle of retreat, or
B. have no effect on it whatsoever because the Blade is fastened at the center,
now whilst the wind is trying to push that retreating blade what is it doing to the advancing blade is it:-
A. trying to slow it down, or
B. make no difference because of the blade being joined at the center,

If this wind input affects either advancing or retreating blades will that in itself impart any longitudenal bending to the advancing blade( similar to the bending that occurs when you sweep a stick of wood thu H2o) if this bending takes placewhich part of the length of the blade is then at risk from most cycles of movement, I would assume any direction blade would come under this stress , and would the stress be greater in a two bade rotor, rather than a three or a four blade assembly.
Not being Degree Educated I cannot work this out can you help?
Vfrpilotpb

Let all your landings be little ones!

To: Vfrpilotpb

I don't know if this completely answers you post but, here goes.

Leading and lagging are the result of physical forces called conservation of angular momentum and Coriolis forces which are very often intertwined when an explanation of both are given. The advancing blade leads and the retreating blade lags. I don’t know if the aerodynamic forces acting on the leading and lagging blades can be measured but it can be seen that those forces are not effecting the leading and lagging. The advancing blade has the increased relative wind which would be in opposition to its’ leading and the retreating blade has that same relative wind assisting it. Yet, the advancing blade leads and the retreating blade lags.

It should be noted that all of the leading (and lagging) takes place behind the pure radial position of the blade. That is, if you could diagram a four-blade system the radial position would have all four blade disposed 90-degrees to each other. However there is another mechanical force in play and that is inertia. Inertia causes the blades to move back in relation to the radial position and all leading and lagging takes place around that position. The only time the blades move ahead of the radial position in flight is during autorotation when the blades are driving the power train.

Another point to consider is spanwise bending of the blades, which you brought up in your post, as well as leading and lagging on a two-blade system. To counter the leading and lagging, most two-blade systems have their rotorhead underslung which decreases the angular difference between the driving axis and the driven axis when cyclic is applied. Leading and lagging is only present when cyclic input is made in the control system whether the pilot does it or it is built into the control system.

Spanwise bending is a result of the imperfect design of the rotor blade in that it is difficult to get the chordwise CG and the pitch axis of the blade to be coincident with each other. One of these elements will try to get in line with the other by bending the blade forward on the spanwise axis.

There are several ways of combating spanwise bending. Some blades have stiffeners built into the trailing edge of the blades, while others have drag links, which allow spanwise adjustment while at the same time rigidizing the blade. The most common design device used on articulated rotors is to move the pitch axis forward of the rotational axis.

If you could diagram this you would note that the pitch axis when compared to the rotating axis of the rotorhead is approximately ¼” ahead of the rotating axis. This mechanically brings the chordwise CG into the position it would move to if you had spanwise bending. It is not perfect, but it minimizes spanwise bending.


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The Cat

[This message has been edited by Lu Zuckerman (edited 21 March 2001).]

[This message has been edited by Lu Zuckerman (edited 21 March 2001).]

I've read the question a couple of times, and still come to the same conclusion; you are asking what the effect of wind is going to have on an a/c that is already in forward flight?

If I am correct, then the answer is no effect whatsoever. The a/c is in a moving body of fluid and does not know that this body is moving relative to the ground ie its windy.

If my understanding is wrong, then probably what Lu is saying is on the right track, but I must confess I didn't study it too hard as it was obvious that he had missed out the reference to "wind" in you query.

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Another day in paradise

Vfrpilotpb,

In answer to the first question, as the advancing blade advances (good eh?) it will generate more lift (A/C speed plus Headwind) this will cause it to fly up. This will cause the blade's CofG to move closer to the centre of rotation and thus (to use the classic sice skater spinning example) it will speed up - this is the coriolis effect Lu mentioned. The opposite is true of the retreating blade - it moves down, it's CofG moves out, so it will slow down. Thus the advancing blade leads slightly and the retreating blade lags - not as you imagined it, with the air pushing against it or with it.

As for the bending. I think Lu was describing a torsional bending of the blade about its longitudinal axis. Your question was, I think, asking about a bending of the longitudinal axis in the plane of rotation (or the plane of the relative air flow) such that the tip of the advancing blade lies 'behind' the root. If I'm right and this was the question, I'm afraid I haven't a clue what the answer would be!! But I would imagine the effect, if it exists, would cause a fair old whipping of the blades as they rotate, from 'tip behind' to 'tip inline(/in front?)' to 'tip behind' ....

While Lu is correct in saying that the lead/lag is predominated by Coriolis forces, I would venture to say that the form drag does have a small effect as well. In other words, if you had an infinitely rigid blade and ran it edgewise through the air there would be a slight lag due to the increased drag on the advancing side and the opposite on the retreating side. However, this is just theoretical thought exercise. Real helicopter blades are much more affected by Coriolis forces as the increased speed causes increased lift which leads to flapping...

As for all four blades on a four-bladed head being 90° from one another, are you sure Lu? I was under the impression that they are not, and think Johnson, Leishman and Prouty have diagrams indicating such. Not sure though.

"...in any helicopter we know that the rotor blades are advancing and at the same time retreating, if a helicopter is flying into on the nose headwind say for example 25knts,..."

If that is not asking about the effect of wind, I don't know what is. See above.

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Another day in paradise

To:Kyrilian

If the blades were purely loaded by centrifugal force the blades would be disposed 90-degrees to each other. The inertia of the blade due to its being rotated about the lag hinge will cause the blades to drag behind the radial position so that they are no longer disposed at 90-degrees to each other. All leading and lagging takes place about this blade position.


To: Robbo Jock

When I addressed spanwise bending I was not addressing torsional bending. Torsional bending takes place but it reacts about the pitch change axis. What I was addressing as spanwise bending which can be demonstrated by locking the blade about its’ drag hinge and applying a force to the trailing edge of the blade. The same would be true if you did the forcing on the trailing edge of a two-blade rotor system with the rotor brake turned on.


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The Cat

I am most grateful for the answers offered to my question ,
LU your answer re the lag and this being taken up in the hinge or flapping mechanism does that mean the rotor blade is always rigidly straight at its leading edge( which in theory should be the strongest part) upon advancment and retreating, or could there be any minute bending ie, rotor tip slightly beind on advance but straightening on retreat, if this is the case the flexing and bending of the rotor leading edge would be the same as bending a spoon backwards and forwards until somthing breaks because of the weakening of the grain structure of the leading edge, now if you take the metal used in most leading edges it is normally die drawn or mould pressed, if it is die drawn then the grain is running laterally from Rotor hub tho blade tip this in theory is the weakest form of granular structure for 90dg stress at he leading edge, I know this must sound like a lost cause but has any research been carried out but blade manufacturers to look at this sort of stress. again I must than all for good answers and ask you guys out there for more help.
My Regards
Peter R-B

To: Vfrpilotpb

As I had indicated previously blade design on paper or on a CAD program is precise. However, when the blades are constructed the positioning of the chordwise CG is not very precise. Most blades have movable weights in the tips and these serve two purposes. 1) They establish chordwise balance in that they can be moved on the whirl stand to compensate for climbing or diving and 2) The weights establish the spanwise CG of the blade mass.

On the ideal blade the chordwise CG would be on the pitch change axis but in reality it is slightly behind that axis. During flight the blade bends spanwise so that the chordwise CG is on the pitch change axis. As previously stated most helicopter manufacturers will design the rotorhead so that the pitch axis is slightly ahead of the rotating axis of the spinning rotor mass. This moves the chordwise CG to the point it wants to be. However even this cannot compensate for manufacturing inaccuracies so that the blade is bent spanwise at all times during its’ rotation and it doesn’t matter if the blade is advancing or retreating. The mechanical forces that cause the bending are independent of the forces that cause leading and lagging.

There have been many incidents where a blade came off in flight but it was due to one of several reasons-which include, manufacturing error, design error or, faulty overhaul but I have never heard of a blade failure due to lead/lag or spanwise bending. That is not to say it has never happened.


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The Cat

[This message has been edited by Lu Zuckerman (edited 22 March 2001).]

Thank you Lu and indeed all other relplies, it has been interesting to get this information and indeed all the knowledge from, and in depth thinking from you chaps
My Regards
Peter R-B
Happy Landings

Surely for advancing and retreating to be taking place then the disc must already be being subjected to passage of air through the disc. Any additional wind velocity will just vary the effects accordingly.